Sensitized photopolymerizable compositions and use thereof in lithographic printing plates

ABSTRACT

Improved photopolymerization initiator systems are comprised of a spectral sensitizer that sensitizes in the ultraviolet or visible regions of the spectrum and an N-aryl, O-aryl or S-aryl polycarboxylic acid co-initiator. The improved initiator systems are incorporated in photo-polymerizable compositions containing one or more addition-polymerizable ethylenically-unsaturated compounds to form compositions suitable for the preparation of radiation-sensitive layers in lithographic printing plates adapted to be imagewise-exposed with ultraviolet- or visible-light-emitting lasers such as argon-ion lasers and frequency doubled Nd:YAG lasers. Such plates are able to effectively meet the dual requirements of very high photospeed and very good shelf-life required in computer-to-plate systems.

This is a Divisional Application of application Ser. No. 08/395,352,filed Feb. 28, 1995, now U.S. Pat. No. 5,629,354.

FIELD OF THE INVENTION

This invention relates in general to photopolymerizable compositions andin particular to photopolymerizable compositions of the type which aresuitable for direct digital exposure. More specifically, this inventionrelates to novel photopolymerizable compositions which are especiallyuseful in the manufacture of lithographic printing plates and tolithographic printing plates prepared therefrom which can be imagewiseexposed with ultraviolet- or visible-light-emitting lasers.

BACKGROUND OF THE INVENTION

It is well known in the art relating to the manufacture of lithographicprinting plates to utilize a photopolymerizable composition as theimage-forming layer. Typical photopolymerizable compositions for thispurpose comprise one or more addition-polymerizableethylenically-unsaturated compounds and a photopolymerization initiatorsystem. The addition-polymerizable compounds can be monomers, oligomersor polymers, or mixtures thereof, and frequently the photopolymerizablecomposition will also include a film-forming polymeric binder.

Photopolymerizable compositions of the type which are suitable fordirect digital exposure are of increasing importance to the printingindustry. Such compositions find particular utility in so-called"direct-to-plate" or "computer-to-plate" systems which utilize imagesetters. Practical considerations have resulted invisible-light-emitting lasers, such as low power argon-ion lasers, beingpreferred for use with plate-capable image setters. The need for highwriting speeds, coupled with the constraint of the low-powered lasersfavored in the industry, has resulted in a requirement for printingplates that have very high photosensitivities. However, highphotosensitivity alone is not enough as it is essential that thephotopolymerizable composition also have good shelf-life.

In an effort to meet the dual objectives of high photosensitivity andgood shelf-life, the prior art has proposed the use of a wide variety ofdifferent photopolymerization initiator systems, but has not been fullysuccessful in simultaneously meeting the dual objectives. Among the mosteffective prior art photopolymerization initiator systems are thosewhich comprise a spectral sensitizing dye and an N-aryl-α-amino acid,such as N-phenylglycine (NPG) or a derivative thereof, as aco-initiator. Thus, for example, U.S. Pat. No. 4,278,751 describes aninitiator system comprising NPG and an amine-substituted ketocoumarin;U.S. Pat. Nos. 4,868,092 and 4,965,171 describe an initiator system inwhich the spectral sensitizing dye is a xanthene, coumarin ormerocyanine and which includes both NPG, or certain derivatives thereof,and a diaryliodonium salt; and U.S. Pat. No. 5,153,236 describes aninitiator system which includes an N-aryl-α-amino acid, such as NPG, andin which the spectral sensitizing dye is a thioxanthone, anisoalloxanine or a coumarin. Photopolymerizable compositions containingan initiator system comprising a coumarin, an iodonium salt and NPG arealso described in J. P. Fouassier et al, "A New Three-Component SystemIn Visible Laser Light Photo-Induced Polymerization", Journal Of ImagingScience And Technology, Vol. 37, No. 2, March/April, 1993.

In addition to the use of N-aryl-α-amino acids as co-initiators, it isalso known to employ O-aryl and S-aryl compounds as described forexample, in U.S. Pat. No. 4,939,069, issued Jul. 3, 1990.

Photoinitiating systems of the bimolecular type consisting of a radicalgenerator and a sensitizing dye and mechanisms whereby they are believedto function are described in Yamaoka et al,"N-Phenylglycine-(Thio)xanthene Dye Photoinitiating System andApplication to Photopolymer for Visible Laser Exposure", Journal ofApplied Polymer Science, Vol. 38, 1271-1285 (1989). As pointed out byYamaoka et al, bimolecular type photoinitiators consisting ofN-phenylglycine and a xanthene or thioxanthene dye exhibit highinitiating efficiency. However, such systems are deficient in regard toshelf-life.

Fluorone dyes are shown to work well as visible light photoinitiators incombination with NPG and iodonium salts in an article by Thomas L.Marino et al, entitled "Chemistry And Properties Of Novel FluoroneVisible Light Photoinitiators" published in the Proceedings of theRadtech '94 Conference, Vol. 1 page 1691, 1994 by Radtech InternationalNorth America. The fluorone dyes described provide panchromaticabsorption throughout the visible spectrum.

Use of NPG as a co-initiator in a photopolymerization initiator systemprovides excellent photospeed, but the composition tends to rapidly losephotospeed on aging, i.e., its shelf-life is poor. Thus, for example, D.F. Eaton has noted that the improvements in photosensitivity provided byamine activators in general and by NPG in particular are lost withindays of preparation of photosensitive films containing such materials.See "Dye Sensitized Photopolymerization" by D. F. Eaton in Advances InPhotochemistry, Vol. 13, D. Volman, K. Gollnick and G. S. Hammond, eds.,Wiley Interscience (1986).

Arimatsu, U.S. Pat. No. 5,378,579, describes a photopolymerizablecomposition in which the NPG co-initiator has been derivatized with acompound such as glycidyl methacrylate in an effort to improveshelf-life. Such use of glycidyl methacrylate is, however, highlyundesirable from an environmental standpoint, because of concernsrelating to toxicity, and improvement in shelf-life is marginal at best.

Additional references relating to the use of iodonium salts inphotopolymerizable compositions include Japanese Patent No. 1,660,964(Patent Application No. 58-198085, filed Oct. 21, 1983, KOKAI No.60-88005, published May 17, 1985, KOKOKU No. 3-62162, published Sep. 25,1991) which describes a photopolymerization initiator system comprisinga 3-ketocoumarin and a diaryl iodonium salt; U.S. Pat. No. 4,921,827,issued May 1, 1990, which describes a photopolymerization initiatorsystem comprising at least one merocyanine sensitizer containing aconstrained alkylamino group and an initiator selected from the groupconsisting of photosensitive iodonium salts, sulfonium salts andhalomethyl s-triazines; and European Patent Application No. 0 290 133which describes a photopolymerization initiator system comprising anaryliodonium salt, a sensitizer and an electron donor having anoxidation potential that is greater than zero and less than or equal tothat of p-dimethoxybenzene.

While the photopolymerizable compositions of the prior art meet many ofthe needs of the lithographic printing plate industry, they still do notfully satisfy the dual requirements of high photospeed and goodshelf-life that are urgently needed. It is toward the objective ofbetter meeting these dual requirements that the present invention isdirected.

SUMMARY OF THE INVENTION

In accordance with this invention, a novel photopolymerizablecomposition, suitable for use in the preparation of lithographicprinting plates, is comprised of:

(1) at least one addition-polymerizable ethylenically-unsaturatedcompound, and

(2) a photopolymerization initiator system comprising:

(A) a spectral sensitizer that sensitizes in the ultraviolet or visibleregions of the spectrum, and

(B) a polycarboxylic acid comprising an aromatic moiety substituted witha hetero atom selected from nitrogen, oxygen and sulfur and comprisingat least two carboxyl groups with at least one of the carboxyl groupsbeing linked to the hetero atom.

The polycarboxylic acids utilized in this invention as co-initiatorscomprise an aromatic moiety. By the term "aromatic moiety", as usedherein, is meant a mono- or polynuclear hydrocarbon radical, such as abenzene, biphenyl, naphthalene or anthracene group. The aromatic moietyis substituted with a nitrogen, oxygen or sulfur atom so that thecompounds can be referred to as N-aryl, O-aryl or S-aryl polycarboxylicacids. In addition to being substituted with a nitrogen, oxygen orsulfur atom, the aromatic moiety can optionally include one or moreadditional substituents.

The N-aryl polycarboxylic acids comprise an N-aryl nucleus which can berepresented by the formula:

    Ar--N<

wherein Ar is a substituted or unsubstituted aryl group. Similarily, theO-aryl polycarboxylic acids comprise an O-aryl nucleus which can berepresented by the formula:

    Ar--O--

and the S-aryl polycarboxylic acids comprise an S-aryl nucleus which canbe represented by the formula:

    Ar--S--

The polycarboxylic acids utilized in this invention include within theirstructure at least two carboxyl groups with at least one of the carboxylgroups being linked to the hetero atom. Thus, the O-aryl and S-arylpolycarboxylic acids have a carboxyl group linked to the oxygen orsulfur atom, respectively, and one or more carboxyl groups linked to theAr group.

The N-aryl polycarboxylic acids can have one carboxyl group linked tothe nitrogen atom and one or more carboxyl groups linked to the Argroup, or they can have two carboxyl groups linked to the nitrogen atomand no carboxyl groups linked to the Ar group, or they can have twocarboxyl groups linked to the nitrogen atom and one or more carboxylgroups linked to the Ar group. When the nitrogen atom is substitutedwith only one carboxyl group, the other substituent on the nitrogen atomcan be hydrogen or a monovalent organic radical such as an alkyl group.

Whether they are N-aryl, O-aryl or S-aryl compounds, the co-initiatorsof this invention are polycarboxylic acids. By contrast, theN-aryl-α-amino carboxylic acids, such as NPG, which have heretofore beenutilized as co-initiators are monocarboxylic acids.

In the present invention, it is preferred that the heteroatom be anitrogen atom and, accordingly, that the polycarboxylic acid be anN-aryl polycarboxylic acid and particularly preferred to utilize anN-aryl polycarboxylic acid in which at least one carboxyl group islinked to the nitrogen atom of the N-aryl nucleus by means of amethylene (--CH₂ --) linking group.

In one preferred embodiment of the invention, the co-initiator is apolycarboxylic acid of the formula: ##STR1## wherein Ar is a substitutedor unsubstituted aryl group and n is an integer with a value of from 1to 5.

In a second preferred embodiment of the invention, the co-initiator is apolycarboxylic acid of the formula: ##STR2## wherein n is an integerwith a value of from 1 to 5, m is an integer with a value of from 1 to5, and

R is hydrogen or an alkyl group of 1 to 6 carbon atoms.

In a third preferred embodiment of the invention, the co-initiator is adicarboxylic acid of the formula: ##STR3## wherein n is an integer witha value of from 1 to 5.

In a fourth preferred embodiment of the invention, the co-initiator is adiacetic acid of the formula: ##STR4## wherein Ar is a substituted orunsubstituted aryl group.

In a fifth preferred embodiment of the invention, the co-initiator is adiacetic acid of the formula: ##STR5## wherein Q¹ and Q² areindependently hydrogen, alkyl, alkoxy, thioalkyl, aryl, aryloxy orhalogen.

In the most preferred embodiment of the invention, the co-initiator isanilinediacetic acid which has the formula: ##STR6## This compound canalso be referred to as N-phenyliminodiacetic acid or asN-(carboxymethyl)-N-phenylacetic acid.

The term "photopolymerization" is used herein in a broad sense toinclude photocrosslinking and photodimerization of macromolecules aswell as to describe the photoinduced initiation of vinyl polymerization.Thus, the photopolymerizable compositions in which the novelphotopolymerization initiator system of this invention is useful canvary widely in composition and properties.

As hereinabove described, it is well known in the art to use thecompound N-phenylglycine (NPG) which has the formula: ##STR7## as aco-initiator in a photopolymerization initiator system comprising aspectral sensitizer that sensitizes in the ultraviolet or visibleregions of the spectrum. However, the co-initiators of this inventionare much more effective than N-phenylglycine in meeting the dualobjectives of high photosensitivity and good shelf-life.

N-aryl-α-amino carboxylic acids, such as NPG, which have heretofore beenproposed for use as co-initiators in photopolymerization initiatorsystems are monocarboxylic acids. In marked contrast, the co-initiatorsof this invention are polycarboxylic acids which comprise two or morecarboxyl groups. The presence of the additional carboxyl substitutionhas been found, most unexpectedly, to contribute greatly to meeting thedual requirements of high photospeed and good shelf-life. Whileapplicants do not intend to be bound by any theoretical explanation ofthe manner in which their invention functions, it is believed that theimproved performance may be related to the ability of the polycarboxylicacid co-initiator to resist thermal decarboxylation.

DETAILED DESCRIPTION OF THE INVENTION

In a particular embodiment, the present invention is directed to a novelphotopolymerization initiator system comprising (A) a spectralsensitizer that sensitizes in the ultraviolet or visible regions of thespectrum and (B) a polycarboxylic acid co-initiator as herein described.

In a further particular embodiment, the present invention is directed toa novel photopolymerizable composition comprising (1) at least oneaddition-polymerizable ethylenically-unsaturated compound and (2) aphotopolymerization initiator system comprising (A) a spectralsensitizer that sensitizes in the ultraviolet or visible regions of thespectrum and (B) a polycarboxylic acid co-initiator as herein described.

In a still further embodiment, the present invention is directed to anovel lithographic printing plate comprising a support having thereon aradiation-sensitive layer adapted to form a lithographic printingsurface in which the radiation-sensitive layer is comprised of theaforesaid novel photopolymerizable composition.

In yet another embodiment, the present invention is directed to a novelprocess of forming a lithographic printing surface comprising the stepsof:

(I) providing a lithographic printing plate comprising a support havingthereon a radiation-sensitive layer comprised of the aforesaid novelphotopolymerizable composition;

(II) imagewise exposing the radiation-sensitive layer of suchlithographic printing plate with an ultraviolet- orvisible-light-emitting laser, and

(III) subjecting the imagewise-exposed lithographic printing plate todevelopment to remove the non-exposed areas of the radiation-sensitivelayer and reveal the underlying support.

The improved photopolymerizable compositions of this invention compriseat least three essential components, namely, an addition-polymerizableethylenically-unsaturated compound, a spectral sensitizer thatsensitizes in the ultraviolet or visible regions of the spectrum, and apolycarboxylic acid co-initiator as herein described. A wide variety ofother ingredients can optionally be included in the composition toimprove its performance.

Any of a very broad range of addition-polymerizableethylenically-unsaturated compounds can be utilized in this invention.Examples include acrylates, methacrylates, acrylamides, methacrylamides,allyl compounds, vinyl ethers, vinyl esters, N-vinyl compounds,styrenes, itaconates, crotonates, maleates and the like. Theaddition-polymerizable ethylenically-unsaturated compound can be amonomer, a pre-polymer such as a dimer, trimer or other oligomer, ahomopolymer, a copolymer, or mixtures thereof.

Acrylic compounds are especially useful as the addition-polymerizableethylenically unsaturated compound. Useful acrylic compounds includemono-functional monomers and polyfunctional monomers. Examples ofmonofunctional acrylic monomers that are useful in the compositions ofthis invention include acrylic and methacrylic esters such as ethylacrylate, butyl acrylate, 2-hydroxypropyl acrylate, cyclohexyl acrylate,2-ethylhexyl acrylate, methyl methacrylate, ethyl methacrylate, and thelike. Examples of polyfunctional acrylic monomers that are usefulinclude:

neopentylglycol diacrylate

pentaerythritol triacrylate

1,6-hexanediol diacrylate

trimethylolpropane triacrylate

tetraethylene glycol diacrylate

1,3-butylene glycol diacrylate

trimethylolpropane trimethacrylate

1,3-butylene glycol dimethacrylate

ethylene glycol dimethacrylate

pentaerythritol tetraacrylate

tetraethylene glycol dimethacrylate

1,6-hexanediol dimethacrylate

ethylene glycol diacrylate

diethylene glycol diacrylate

glycerol diacrylate

glycerol triacrylate

1,3-propanediol diacrylate

1,3-propanediol dimethacrylate

1,2,4-butanetriol trimethacrylate

1,4-cyclohexanediol diacrylate

1,4-cyclohexanediol dimethacrylate

pentaerythritol diacrylate

1,5-pentanediol dimethacrylate, and the like.

Preferred polyfunctional acrylic monomers are those of the formula:##STR8## wherein each R¹ is independently selected from the groupconsisting of a hydrogen atom and an alkyl group of 1 to 2 carbon atoms,and each R² is independently selected from the group consisting of analkyl group of 1 to 6 carbon atoms and a radical of the formula:##STR9## in which R³ is a hydrogen atom or an alkyl group of 1 to 2carbon atoms.

Poly(alkylene glycol)diacrylates are also especially useful in thepresent invention as the addition-polymerizableethylenically-unsaturated compound. Examples are compounds representedby the following formula: ##STR10## wherein R is an alkylene radical of2 to 4 carbon atoms, R₁ and R₂ are independently H or CH₃, and x is aninteger having a value of from 3 to 50.

Included among the useful poly(alkylene glycol) diacrylates are thosecomprising ethyleneoxy, propyleneoxy or butyleneoxy groups. As indicatedby the above formula, the terminal groups can be acrylate ormethacrylate groups.

The photopolymerizable compositions of this invention can include aphotocrosslinkable polymer. Typical of such photocrosslinkable polymersare those containing the photosensitive group --CH═CH--CO--as anintegral part of the polymer backbone, especially the p-phenylenediacrylate polyesters. These polymers are described, for example, inU.S. Pat. Nos. 3,030,208, 3,622,320, 3,702,765 and 3,929,489. A typicalexample of such a photocrosslinkable polymer is the polyester preparedfrom diethyl p-phenylenediacrylate and1,4-bis(β-hydroxyethoxy)cyclohexane.

An essential component of the photo-polymerizable compositions of thisinvention is a spectral sensitizer that sensitizes in the ultraviolet orvisible regions of the spectrum. Any of a very broad range of spectralsensitizers can be used for this purpose. Examples of preferred spectralsensitizers are coumarins, fluoresceins, fluorones, xanthenes,merocyanines, thioxanthones, isoalloxanines and the like.

Particularly preferred spectral sensitizers for use in this inventionare 3-substituted coumarins as described in Specht et al, U.S. Pat. No.4,147,552, issued Apr. 3, 1979. These coumarins have an absorptivemaximum between about 250 and about 550 nm. As described in the '552patent, they can be represented by the formula: ##STR11## wherein Q is--CN or --Z--R¹ ; Z is carbonyl, sulfonyl, sulfinyl orarylenedicarbonyl;

R¹ is alkenyl; alkyl having 1-12 carbon atoms, aryl of 6-10 nuclearcarbon atoms; a carbocyclic group of 5-12 carbon atoms; or aheterocyclic group having 5-15 nuclear carbon and hetero atoms;

R², R³, R⁴ and R⁵ each independently is hydrogen, alkoxy having 1-6carbon atoms, dialkylamino with each alkyl of the dialkylamino grouphaving 1-4 carbon atoms, halogen, acyloxy, nitro, a 5- or 6-memberedheterocyclic group, or a group having the formula: ##STR12## wherein R⁹is an alkylene radical having from 1-5 carbon atoms; R⁶ is hydrogen,alkyl having 1-4 carbon atoms, aryl of 6-10 carbon atoms; and

wherein two or three of R² -R⁵ and the nuclear carbon atoms to whichthey are attached can together form a fused ring or fused ring system,each ring being a 5- to 6-membered ring.

Typical examples of useful 3-substituted coumarins include:

3-benzoyl-5,7-dimethoxycoumarin

3-benzoyl-7-methoxycoumarin

3-benzoyl-6-methoxycoumarin

3-benzoyl-8-ethoxycoumarin

7-methoxy-3-(p-nitrobenzoyl)coumarin

3-benzoylcoumarin

3-(p-nitrobenzoyl)coumarin

3-benzoylbenzo f!coumarin

3,3'-carbonylbis(7-methoxycoumarin)

3-acetyl-7-methoxycoumarin

3-benzoyl-6-bromocoumarin

3,3'-carbonylbiscoumarin

3-benzoyl-7-dimethylaminocoumarin

3,3'-carbonylbis(7-diethylaminocoumarin)

3-carboxycoumarin

3-carboxy-7-methoxycoumarin

3-methoxycarbonyl-6-methoxycoumarin

3-ethoxycarbonyl-6-methoxycoumarin

3-ethoxycarbonyl-7-methoxycoumarin

3-methoxycarbonyl-7-methoxycoumarin

3-acetylbenzo f!coumarin

3-acetyl-7-methoxycoumarin

3-(1-admantoyl)-7-methoxycoumarin

3-benzoyl-7-hydroxycoumarin

3-benzoyl-6-nitrocoumarin

3-benzoyl-7-acetoxycoumarin

3- 3-(p-ethoxyphenyl)acryloyl!-7-methoxycoumarin

3-benzoyl-7-diethylaminocoumarin

7-dimethylamino-3-(4-iodobenzoyl)coumarin

7-diethylamino-3-(4-iodobenzoyl)coumarin

3,3'-carbonylbis(5,7-diethoxycoumarin)

3-(2-benzofuroyl)-7-(1-pyrrolidinyl)coumarin

7-diethylamino-3-(4-dimethylaminobenzoyl)coumarin

7-methoxy-3-(4-methoxybenzoyl)coumarin

3-(4-nitrobenzoyl)benzo f!coumarin

3-(4-ethoxycinnamoyl)-7-methoxycoumarin

3-(4-dimethylaminocinnamoyl)coumarin

3-(4-diphenylaminocinnainoyl)coumarin

3- (3-methylbenzothiaz!-2-ylidene)acetyl!coumarin

3- (1-methylnaphtho 1,2-d!thiazol-2-ylidene)acetyl!-coumarin

3,3'-carbonylbis(6-methoxycoumarin)

3,3'-carbonylbis(7-acetoxycoumarin)

3,3'-carbonylbis(7-dimethylaminocoumarin)

3,3'-carbonylbis(5,7-di-isopropoxycoumarin)

3,3'-carbonylbis(5,7-di-n-propoxycoumarin)

3,3'-carbonylbis(5,7-di-n-butoxycoumarin)

3,3'-carbonylbis 5,7-di(2-phenylethoxy)coumarin!

3,3'-carbonylbis 5,7-di(2-chloroethoxy)coumarin!

3-cyano-6-methoxycoumarin

3-cyano-7-methoxycoumarin

7-methoxy-3-phenylsulfonylcoumarin

7-methoxy-3-phenylsulfinylcoumarin

1,4-bis(7-diethylamino-3-coumarylcarbonyl)benzene

7-diethylamino-5',7'-dimethoxy-3,3'carbonylbiscoumarin

7-dimethylamino-3-thenoyl coumarin

7-diethylamino-3-furoyl coumarin

7-diethylamino-3-thenoyl coumarin

3-benzoyl-7-(1-pyrrolidinyl)coumarin

3-(4-fluorosulfonyl)benzoyl-7-methoxycoumarin

3-(3-fluorosulfonyl)benzoyl-7-methoxycoumarin

5,7,6'-trimethoxy-3,3'-carbonylbiscoumarin

5,7,7'-trimethoxy-3,3'-carbonylbiscoumarin

7-diethylamino-6'-methoxy-3,3'-carbonylbiscoumarin

3-nicotinoyl-7-methoxycoumarin

3-(2-benzofuroyl)-7-methoxycoumarin

3-(7-methoxy-3-coumarinoyl)-1-methylpyridinium fluorosulfate

3-(5,7-diethoxy-3-coumarinoyl)-1-methylpyridinium fluoroborate

N-(7-methoxy-3-coumarinoylmethyl)pyridinium bromide

3-(2-benzofuroyl)-7-diethylaminocoumarin

7-(1-pyrrolidinyl)-3-thenoylcoumarin

7-methoxy-3-(4-pyridinoyl)coumarin

3,6-dibenzoylcoumarin

N-(7-methoxy-3-coumarinoylmethyl)-N-phenylacetamide and

9-(7-diethylamino-3-coumarinoyl)-1,2,4,5-tetrahydro-3H,6H,10H1!benzopyrano 9,9a,1-gh!quinolazine-10-one.

Of the coumarin sensitizers, the 3-ketocoumarins are especiallypreferred for use in this invention.

Among the wide variety of sensitizers suitable for use in thisinvention, the merocyanine sensitizers containing a constrainedalkylamino group are particularly preferred because of their ability toprovide high photospeed. Such sensitizers are described, for example, inU.S. Pat. No. 4,921,827 which defines a constrained alkylamino group asa saturated heterocyclic structure containing at least one nitrogen atomwhich is directly attached to an existing aromatic ring of themerocyanine.

A preferred sensitizer for use in the present invention is10-(3-(4-diethylamino)phenyl)-1-oxo-2-propenyl)-2,3,6,7-tetrahydro-1H,5H,11H-(1)benzopyrano(6,7,8-ij)quinolizin-11-onewhich has the formula: ##STR13## This sensitizer is described in U.S.Pat. Nos. 4,289,844 and 4,366,228, is identified as "Sensitizer A" inU.S. Pat. No. 4,921,827 and is referred to hereinafter as Sensitizer A.

Another preferred sensitizer for use in the present invention is3,3'-carbonylbis(7-diethylaminocoumarin) which has the formula:##STR14## This sensitizer and its synthesis are described in U.S. Pat.No. 4,147,552. It is referred to hereinafter as Sensitizer B.

It is advantageous to use mixtures of spectral sensitizers in thisinvention and a particularly useful mixture is a mixture of Sensitizer Aand Sensitizer B. Sensitizer B is particularly effective when it isdesired to expose with an argon-ion laser that emits at 488 nanometersbut is not effective for use with a frequency doubled Nd:YAG laser thatemits at 532 nanometers. Sensitizer A is equally effective at 488nanometers and 532 nanometers but its efficiency is significantly lessthan that of Sensitizer B. By using a mixture of Sensitizers A and B,optimum results are obtained whether the chosen exposure source is anargon-ion laser or a frequency doubled Nd:YAG laser.

As hereinabove described, the photopolymerization initiator system ofthis invention includes a spectral sensitizer that sensitizes in theultraviolet or visible regions of the spectrum and a polycarboxylic acidco-initiator comprising an aromatic moiety substituted with a heteroatom selected from nitrogen, oxygen and sulfur and comprising at leasttwo carboxyl groups with at least one of the carboxyl groups beinglinked to the hetero atom. N-aryl polycarboxylic acid co-initiators areespecially effective in this invention and examples of preferred N-arylpolycarboxylic acid co-initiators include those of the followingformulae I, II, III, IV and V: ##STR15## wherein Ar is a substituted orunsubstituted aryl group and n is an integer with a value of from 1 to5. ##STR16## wherein n is an integer with a value of from 1 to 5, m isan integer with a value of from 1 to 5, and

R is hydrogen or an alkyl group of 1 to 6 carbon atoms. ##STR17##wherein n is an integer with a value of from 1 to 5. ##STR18## whereinAr is a substituted or unsubstituted aryl group. ##STR19## wherein Q¹and Q² are independently hydrogen, alkyl, alkoxy, thioalkyl, aryl,aryloxy or halogen.

Preferably Q¹ and Q² are independently selected from the groupconsisting of hydrogen, methyl, ethyl, propyl, butyl, methoxy, ethoxy,propoxy, butoxy, thiomethyl, thioethyl and chloro.

Specific examples of N-aryl, O-aryl and S-aryl polycarboxylic acids thatcan be used as co-intiators in this invention include:

anilinediacetic acid

(p-acetamidophenylimino)diacetic acid

3-(bis(carboxymethyl)amino)benzoic acid

4-(bis(carboxymethyl)amino)benzoic acid

2- (carboxymethyl)phenylamino!benzoic acid

2- (carboxymethyl)methylamino!benzoic acid

2- (carboxymethyl)methylamino!-5-methoxybenzoic acid

3- bis(carboxymethyl)amino!-2-naphthalenecarboxylic acid

N-(4-aminophenyl)-N-(carboxymethyl)glycine

N,N'-1,3-phenylenebisglycine

N,N'-1,3-phenylenebis N-(carboxymethyl)!glycine

N,N'-1,2-phenylenebis N-(carboxymethyl)!glycine

N-(carboxymethyl)-N-(4-methoxyphenyl)glycine

N-(carboxymethyl)-N-(3-methoxyphenyl)glycine

N-(carboxymethyl)-N-(3-hydroxyphenyl)glycine

N-(carboxymethyl)-N-(3-chlorophenyl)glycine

N-(carboxymethyl)-N-(4-bromophenyl)glycine

N-(carboxymethyl)-N-(4-chlorophenyl)glycine

N-(carboxymethyl)-N-(2-chlorophenyl)glycine

N-(carboxymethyl)-N-(4-ethylphenyl)glycine

N-(carboxymethyl)-N-(2,3-dimethylphenyl)glycine

N-(carboxymethyl)-N-(3,4-dimethylphenyl)glycine

N-(carboxymethyl)-N-(3,5-dimethylphenyl)glycine

N-(carboxymethyl)-N-(2,4-dimethylphenyl)glycine

N-(carboxymethyl)-N-(2,6-dimethylphenyl)glycine

N-(carboxymethyl)-N-(4-formylphenyl)glycine

N-(carboxymethyl)-N-ethylanthranilic acid

N-(carboxymethyl)-N-propylanthranilic acid

5-bromo-N-(carboxymethyl)anthranilic acid

N-(2-carboxyphenyl)glycine

o-dianisidine-N,N,N',N'-tetraacetic acid

N,N'- 1,2-ethanediylbis(oxy-2,1-phenylene)!bis N-(carboxymethyl)glycine!

4-carboxyphenoxyacetic acid

catechol-O,O'-diacetic acid

4-methylcatechol-O,O'-diacetic acid

resorcinol-O,O'-diacetic acid

hydroquinone-O,O'-diacetic acid

α-carboxy-o-anisic acid

4,4'-isopropylidenediphenoxyacetic acid

2,2'-(dibenzofuran-2,8-diyldioxy)diacetic acid

2-(carboxymethylthio)benzoic acid

5-amino-2-(carboxymethylthio)benzoic acid

3- (carboxymethyl)thio!-2-naphthalenecarboxylic acid

In addition to the spectral sensitizer and polycarboxylic acidco-initiator, it is particularly preferred to include in thephotopolymerization initiator system of this invention a secondco-initiator selected from the group consisting of (1) iodonium salts,(2) titanocenes, (3) haloalkyl-substituted s-triazines, (4) hexaarylbisimidazoles, (5) photooxidants containing a heterocyclic nitrogen atomthat is substituted by either an alkoxy group or an acyloxy group suchas, for example, the N-alkoxypyridinium salts, and (6)alkyltriarylborate salts. The use of one or more of the aforesaid secondco-initiators improves the performance of the photopolymerizablecomposition in terms of meeting the dual requirements of high photospeedand good shelf-life.

Preferred iodonium salts for use in this invention are iodonium salts ofthe formula: ##STR20## in which Ar¹ and Ar² independently representsubstituted or unsubstituted aromatic groups and X⁻ represents an anion.

Particularly preferred iodonium salts for use in this invention arediaryl iodonium salts of the formula: ##STR21## wherein each R isindependently hydrogen, halogen, alkyl, alkoxy or nitro and X is ananion such as Cl⁻, BF₄ ⁻, PF₆ ⁻, AsF₆ ⁻ or SbF₆ ⁻.

Specific examples of iodonium salts that are useful in this inventioninclude:

diphenyliodonium hexafluorophosphate

4-octyloxyphenylphenyliodonium hexafluorophosphate

bis(dodecylphenyl)iodonium hexafluoroantimonate

4-isooctyloxyphenylphenyliodonium hexafluorophosphate,

diphenyliodonium trifluoromethanesulfonate

4-octyloxyphenylphenyliodonium tosylate

4,4'-di-t-butyl-diphenyliodonium hexafluorophosphate

4-butoxyphenylphenyliodonium trifluoroacetate

diphenyliodonium naphthalenesulfonate

3,3'-dinitrodiphenyliodonium hexafluorophosphate

4,4'-dichlorodiphenyliodonium tosylate

4-methoxyphenylphenyliodonium tetrafluoroborate and the like.

The use of iodonium salts in combination with spectral sensitizers toform a photopolymerization initiator system is well known in the art.References disclosing such combinations are discussed herein in thesection entitled "Background Of The Invention." Such references includeJapanese Patent No. 1,660,964 (KOKOKU 3-62162) assigned to the Agencyfor Industrial Science and Technology, which describesphotopolymerization initiator systems comprising a 3-ketocoumarin and adiaryl iodonium salt, European Patent Application No. 0 290 133,published Nov. 9, 1988, which describes photopolymerization initiatorsystems comprising a spectral sensitizer, an aryliodonium salt and anelectron donor having an oxidation potential that is greater than zeroand less than or equal to that of p-dimethoxybenzene; and U.S. Pat. No.4,921,827 which describes photopolymerization initiator systemscomprising a merocyanine sensitizer containing a constrained alkylaminogroup and a co-initiator selected from diaryl iodonium salts,halogenated triazines and triaryl sulfonium salts. However, it was notheretofore known to include an N-aryl, O-aryl or S-aryl polycarboxylicacid as a co-initiator in a photopolymerization initiator systemcontaining both a spectral sensitizer and an iodonium salt.

In marked contrast with the benefits provided by the use in thisinvention of iodonium salts as a second co-initiator, use of sulfoniumor diazonium salts for this purpose has been found to bring about nochange in photospeed or a reduction in photospeed.

Titanocenes which are especially useful as a second co-initiator in thephotopolymerization initiator system of this invention include thosedescribed in U.S. Pat. No. 5,306,600. Of particular interest is thetitanocene compoundbis(η5-2,4-cyclopentadienyl-1-yl)-bis{2,6-difluoro-3-(1H-pyrrol-1-yl)phenyl}titaniumwhich has the following formula: ##STR22##

Another class of compounds that are useful as a second co-initiator inthe present invention are the photooxidants described in U.S. Pat. No.Reissue 28,240, the disclosure of which is incorporated herein byreference. These are photooxidants which contain a heterocyclic nitrogenatom that is substituted by either an alkoxy group or an acyloxy group.As described in U.S. Pat. No. Re 28,240 typical photooxidants of thisclass are represented by one of the formulas: ##STR23## wherein R₁ canbe any of the following:

a. a methine linkage terminated by a heterocyclic nucleus of the typecontained in cyanine dyes, e.g., those set forth in Mees and James, "TheTheory of the Photographic Process," Macmillan, 3rd ed, pp. 198-232; themethine linkage can be substituted or unsubstituted, e.g., --CH═,--C(CH₃)═, --C(C₆ H₅)═, --CH═CH--, --CH═CH--CH═, etc.;

b. an alkyl radical preferably containing one to eight carbon atomsincluding a substituted alkyl radical;

c. an aryl radical including a substituted aryl radical such as a phenylradical, a naphthyl radical, a tolyl radical, etc.;

d. a hydrogen atom;

e. an acyl radical having the formula: ##STR24## wherein R₉ is hydrogenor an alkyl group preferably having one to eight carbon atoms;

f. an anilinovinyl radical such as a radical having the formula:##STR25## wherein R₃ is hydrogen, acyl or alkyl, or g. a styryl radicalincluding substituted styryl radicals, e.g., ##STR26## wherein R₂ ishydrogen, alkyl, aryl, amino, including dialkylamino such asdimethylamino;

R₈ can be either of the following:

a. a methine linkage terminated by a heterocyclic nucleus of the typecontained in merocyanine dyes, e.g., those set forth in Mees and James(cited above); the methine linkage can be substituted or unsubstituted;or

b. an allylidene radical including a substituted allylidene radical suchas a cyanoallylidene radical, an alkylcarboxyallylidene radical or analkylsufonylallylidene radical;

R can be either:

a. an alkyl radical preferably having one to eight carbon atoms such asmethyl, ethyl, propyl, butyl, etc., including a substitued alkyl radicalsuch as sulfoalkyl, e.g., --(CH₂)₃ SO₃ --, an aralkyl, e.g., benzyl orpyridinatooxyalkyl salt, e.g., --(CH₂)₃ --O--Y wherein Y is substitutedor unsubstituted pyridinium salt; etc.,

b. an acyl radical, e.g., ##STR27## wherein R₄ is an alkyl radicalpreferably having one to eight carbon. atoms or aryl radical, e.g.,methyl, ethyl, propyl, butyl, phenyl, naphthyl, etc.

Z represents the atoms necessary to complete a five- to six-memberedheterocyclic nucleus, which nucleus can contain at least one additionalheteroatom such as oxygen, sulfur, selenium or nitrogen, e.g., apyridine nucleus, a quinoline nucleus, etc.; and

X⁻ represents an acid anion, e.g, chloride, bromide, iodide,perchlorate, sulfamate, thiocyanate, p-toluenesulfonate, methyl sulfate,tetrafluoroborate, etc.

Examples of photooxidants of the class described in Reissue U.S. Pat.No. Re 28,240 which are especially preferred for use in this inventioninclude: ##STR28##

Of the photooxidants of Reissue U.S. Pat. No. Re 28,240, those that aremost preferred for use in the present invention are theN-alkoxypyridinium salts.

Haloalkyl-substituted s-triazines are also useful as a secondco-initiator in the present invention. Preferred haloalkyl-substituteds-triazines for use in this invention are compounds of the formula:##STR29## wherein R₁ is a substituted or unsubstituted aliphatic oraromatic radical and R₂ and R₃ are, independently, haloalkyl groups.

In the above formula, it is especially preferred that R₂ and R₃ arehaloalkyl groups of 1 to 4 carbon atoms.

Particularly preferred haloalkyl-substituted s-triazines for use in thisinvention are compounds of the formula: ##STR30## wherein R₁ is asubstituted or unsubstituted aliphatic or aromatic radical and each Xis, independently, a halogen atom.

A specific example of a preferred haloalkyl-substituted s-triazine foruse in this invention is 2,4,6-tris(trichloromethyl)-s-triazine.

Hexaaryl bisimidazoles represent a further class of compounds that areuseful as a second co-initiator in the present invention. An example ofa useful hexaaryl bisimidazole is the compound2,2'-bis(o-chlorophenyl)-4,4',5,5'-tetraphenylbisimidazolyl which hasthe formula: ##STR31## wherein Ph represents a phenyl group.

Alkyltriarylborate salts represent a still further class of compoundsthat are useful as a second co-initiator in the present invention. Thealkyltriarylborate salts which are especially useful are salts of theformula: ##STR32## wherein each of R¹, R² and R³ is, independently, anaryl group of 6 to 12 carbon atoms, R⁴ is an alkyl group of 1 to 12carbon atoms, and Y is a counterion such as ammonium,tetramethylammonium or an alkali metal. The three aryl groups arepreferably phenyl or lower alkyl substituted phenyl groups such as tolyland xylyl. The alkyl group is preferably lower alkyl such as methyl,ethyl or n-butyl.

Iodonium salts are particularly useful in this invention because oftheir ability to both substantially improve photospeed and substantiallyimprove shelf-life. Thus, a composition containing both an N-aryl,O-aryl or S-aryl polycarboxylic acid and an iodonium salt represents apreferred embodiment of the invention. Incorporating a titanocene in thecomposition, in addition to the iodonium salt and the polycarboxylicacid, affords additional photospeed which is not obtainable by simplyincreasing the concentration of either the iodonium salt or thepolycarboxylic acid.

Preferred iodonium salts for use in this invention are the non-toxicalkoxy-substituted salts described in U.S. Pat. Nos. 4,882,201,4,981,881, 5,010,118, 5,082,686 and 5,144,051. These compounds arepreferably utilized as the hexafluorophosphate salt rather than thehexafluoroantimonate salt when the lithographic printing plate utilizesan aluminum support. The reason for such preference is that theincompatability of the hexafluoroantimonate counter-ion with thealuminum substrate harms the shelf-life of the plate.

In the photopolymerizable compositions of this invention, the spectralsensitizer is typically used in an amount of from about 0.001 to about0.1 parts per part by weight of the addition-polymerizableethylenically-unsaturated compound, while the polycarboxylic acidco-initiator is typically used in an amount of from about 0.05 to about0.5 parts per part by weight of the addition-polymerizableethylenically-unsaturated compound. When a second co-initiator isemployed, it is typically used in an amount of from about 0.05 to about0.5 parts per part by weight of the addition-polymerizableethylenically-unsaturated compound.

The photopolymerizable compositions of this invention can optionallycontain a film-forming binder. Such use of film-forming binders can beparticularly advantageous in improving the wear resistance of the layerformed therefrom. Useful film-forming binders include styrene-butadienecopolymers; silicone resins; styrene-alkyd resins; silicone-alkydresins; soy-alkyd resins; poly(vinyl chloride); poly(vinylidenechloride); vinylidene chloride-acrylonitrile copolymers; poly(vinylacetate); vinyl acetate-vinyl chloride copolymers; poly(vinyl acetals),such as poly(vinyl butyral); polyacrylic and -methacrylic esters, suchas poly(methyl methacrylate), poly(n-butyl methacrylate) andpoly(isobutyl methacrylate); polystyrene; nitrated polystyrene;polymethylstyrene; isobutylene polymers; polyesters, such aspoly(ethylene-co-alkaryloxyalkylene terephthalate); phenolformaldehyderesin; ketone resins; polyamides; polycarbonates; polythiocarbonates,poly(ethylene 4,4'-isopropylidenediphenylene terephthalate); copolymersof vinyl acetate such as poly(vinyl-m-bromobenzoate-co-vinyl acetate);ethyl cellulose, poly(vinyl alcohol), cellulose acetate, cellulosenitrate, chlorinated rubber and gelatin.

Particular advantages are achieved by the use of acetal polymers as thefilm-forming binder. Examples of useful acetal polymers are thosedescribed in U.S. Pat. Nos. 4,652,604, 4,741,985, 4,940,646, 5,169,897and 5,169,898.

In U.S. Pat. No. 4,652,604, the acetal polymer contains acetal groups ofthree types, namely six-membered cyclic acetals, five-membered cyclicacetals and intermolecular acetals. In U.S. Pat. No. 4,741,985, theacetal polymer is a mono-acetal containing a six-membered cyclic acetalgroup. In U.S. Pat. No. 4,940,646, the acetal polymer contains vinylacetal units derived from an aldehyde that contains hydroxyl groups. InU.S. Pat. No. 5,169,897 the acetal polymer is a binary acetal polymercomprised of recurring units which include two six-membered cyclicacetal groups, one of which is unsubstituted or substituted with analkyl or hydroxyalkyl group and the other of which is substituted withan aromatic or heterocyclic moiety. In U.S. Pat. No. 5,169,898 theacetal polymer is a ternary acetal polymer comprised of recurring unitswhich include three six-membered cyclic acetal groups, one of which isunsubstituted or substituted with an alkyl or hydroxyalkyl group,another of which is substituted with an aromatic or heterocyclic moiety,and a third of which is substituted with an acid group, anacid-substituted alkyl group or an acid-substituted aryl group.

The acid-substituted ternary acetal polymers of U.S. Pat. No. 5,169,898are especially useful in this invention. As described in the '898 patentthese acid-substituted ternary acetal polymers have recurring unitsrepresented by the formula: ##STR33## wherein R₁ is --H, --C_(n)H_(2n+1) or --C_(n) H_(2n) --OH where n=1-12

R₂ is ##STR34## where R₃ is ##STR35## and x=0-8 m=0-8

y=0-8

p=0-8

R₄ =--H, --R₅, ##STR36## in which Y= ##STR37## R₅ =--OH, --CH₂ OH,--OCH₃, --COOH or --SO₃ H z=1 to 3

R₆ =--(CH₂)_(a) --COOH

--(CH₂)_(a) --COO.sup.⊖ M.sup.⊕ ##STR38## where R₇ =--COOH, --COO.sup.⊖M.sup.⊕, --(CH₂)_(a) COOH, --O--(CH₂)_(a) COOH,

--SO₃ H, --S₃.sup.⊖ M.sup.⊕, --PO₃ H₂, --PO₃.sup.⊖ M₂.sup.⊕,

PO₄ H₂ or --PO₄.sup.⊖ M₂.sup.⊕,

a=0 to 8

M=Na, K, Li or NH₄

and n₁ =0-25 mole %, preferably 3 to 15 mole %

n₂ =2-25 mole %, preferably 5 to 15 mole %

n₃ =10-70 mole %, preferably 15 to 50 mole %

n₄ =10-60 mole %, preferably 12 to 45 mole %

n₅ =10-45 mole %, preferably 15 to 30 mole %.

As indicated by the above structural formula, the acid-substitutedternary acetal polymers can be tetramers, in which the recurring unitcomprise a vinyl acetate moiety and first, second and third cyclicacetal groups, or pentamers in which the recurring unit comprises avinyl alcohol moiety, a vinyl acetate moiety and first, second, andthird cyclic acetal groups.

All three of the acetal groups are six-membered cyclic acetal groups,one of them is unsubstituted or substituted with an alkyl orhydroxyalkyl group, another is substituted with an aromatic orheterocyclic moiety, and a third is substituted with an acid group, anacid-substituted alkyl group or an acid-substituted aryl group.

The acid-substituted ternary acetal polymers provide lithographicprinting plates characterized by improved abrasion-resistance, improvedresistance to chemical attack, extended press performance and enhancedroll-up properties in comparison with other acetal polymers.Lithographic printing plates utilizing the acid-substituted ternaryacetal polymers as polymeric binders also have the important advantagethat they can be processed in aqueous alkaline developing solutionscontaining very low concentrations of organic solvents. This is highlyadvantageous in view of the high costs and environmental concernsassociated with the use of organic solvents. Since the acid-substitutedternary acetal polymers are fully soluble in aqueous alkaline developingsolutions, they avoid the problems encountered with binders that causethe coating to break-up in particulate form.

A particularly preferred binder for use in this invention is theacid-substituted ternary acetal polymer described in Example 1 of U.S.Pat. No. 5,169,898, which is referred to hereinafter as binder ATAP.

In the present invention, the combination of diaryl iodonium salt and anN-aryl, O-aryl or S-aryl polycarboxylic acid gives an excellentcombination of high photospeed and long shelf-life and thus represents apreferred embodiment of the invention. The combination of adiphenyliodonium salt and anilinediacetic acid represents a particularlypreferred embodiment. An optimum formulation includes thediphenyliodonium salt, the anilinediacetic acid and binder ATAP.

Lithographic printing plates that are capable of being imagewise exposedwith ultraviolet- or visible-light-emitting lasers can be prepared bycoating the photopolymerizable compositions described herein on asuitable support, and removing the solvent by drying at ambient orelevated temperatures. Any one of a variety of conventional coatingtechniques can be employed, such as extrusion coating, doctor-bladecoating, spray-coating, dip coating, whirl coating, spin coating ,roller coating, etc.

Coating compositions can be prepared by dispersing or dissolving thecomponents in any suitable solvent or combination of solvents. Thesolvents are chosen to be substantially unreactive toward the componentsand are chosen to be compatible with the substrate employed for coating.While the best choice of solvent will vary with the exact applicationunder consideration, exemplary preferred solvents include alcohols, suchas butanol and benzyl alcohol; ketones, such as acetone, 2-butanone andcyclohexanone; ethers such as tetrahydrofuran and dioxane;2-methoxyethyl acetate; N,N'-dimethylformamide; chlorinated hydrocarbonssuch as chloroform, trichloroethane, 1,2-dichloroethane,1,1-dichloroethane, 1,1,2-trichloroethane, dichloromethane,tetrachloroethane, chlorobenzene; and mixtures thereof.

Suitable supports can be chosen from among a variety of materials whichdo not directly chemically react with the coating composition. Suchsupports include fiber-based materials such as paper,polyethylene-coated paper, polypropylene-coated paper, parchment, cloth,etc., sheets and foils of such materials as aluminum, copper, magnesium,zinc, etc.; glass and glass coated with such metals as chromium alloys,steel, silver, gold, platinum, etc.; synthetic resins and polymericmaterials such as poly(alkyl acrylates), e.g., poly(methylmethacrylate), polyester film base, e.g., poly(ethylene terephthalate),poly(vinyl acetals), polyamides, e.g., nylon and cellulose ester filmbase, e.g., cellulose nitrate, cellulose acetate, cellulose acetatepropionate, cellulose acetate butyrate and the like.

Preferred support materials include zinc, anodized aluminum, grainedaluminum, and aluminum which has been grained and anodized.

The support can be preliminarily coated--i.e., before receipt of theradiation-sensitive coating--with known subbing layers such ascopolymers of vinylidene chloride and acrylic monomers--e.g.,acrylonitrile, methyl acrylate, etc. and unsaturated dicarboxylic acidssuch as itaconic acid, etc; carboxymethyl cellulose, gelatin;polyacrylamide; and similar polymer materials.

The optimum coating thickness of the radiation-sensitive layer willdepend upon such factors as the particular application to which theprinting plate will be put, and the nature of other components which maybe present in the coating. Typical coating thicknesses can be from about0.05 to about 10.0 microns or greater, with thicknesses of from 0.1 to2.5 microns being preferred.

Aluminum has been used for many years as a support for lithographicprinting plates and is particularly preferred for use with the printingplates of this invention. In order to prepare the aluminum for such use,it is typical to subject it to both a graining process and a subsequentanodizing process. The graining process serves to improve the adhesionof the subsequently applied radiation-sensitive coating and to enhancethe water-receptive characteristics of the background areas of theprinting plate. The graining affects both the performance and thedurability of the printing plate, and the quality of the graining is acritical factor determining the overall quality of the printing plate. Afine, uniform grain that is free of pits is essential to provide thehighest quality performance.

Both mechanical and electrolytic graining processes are well known andwidely used in the manufacture of lithographic printing plates. Optimumresults are usually achieved through the use of electrolytic graining,which is also referred to in the art as electrochemical graining orelectrochemical roughening, and there have been a great many differentprocesses of electrolytic graining proposed for use in lithographicprinting plate manufacturing. Processes of electrolytic graining aredescribed, for example, in U.S. Pat. Nos. 3,755,116, 3,887,447,3,935,080, 4,087,341, 4,201,836, 4,272,342, 4,294,672, 4,301,229,4,396,468, 4,427,500, 4,468,295, 4,476,006, 4,482,434, 4,545,875,4,538,683, 4,564,429, 4,582,996, 4,618,405, 4,735,696, 4,897,168 and4,919,774.

In the manufacture of lithographic printing plates, the graining processis typically followed by an anodizing process, utilizing an acid such assulfuric or phosphoric acid, and the anodizing process is typicallyfollowed by a process which renders the surface hydrophilic such as aprocess of thermal silication or electrosilication. The anodization stepserves to provide an anodic oxide layer and is preferably controlled tocreate a layer of at least 0.3 g/m². Processes for anodizing aluminum toform an anodic oxide coating and then hydrophilizing the anodizedsurface by techniques such as silication are very well known in the art,and need not be further described herein.

Included among the many patents relating to processes for anodization oflithographic printing plates are U.S. Pat. Nos. 2,594,289, 2,703,781,3,227,639, 3,511,661, 3,804,731, 3,915,811, 3,988,217, 4,022,670,4,115,211, 4,229,266 and 4,647,346. Illustrative of the many materialsuseful in forming hydrophilic barrier layers are polyvinyl phosphonicacid, polyacrylic acid, polyacrylamide, silicates, zirconates andtitanates. Included among the many patents relating to hydrophilicbarrier layers utilized in lithographic printing plates are U.S. Pat.Nos. 2,714,066, 3,181,461, 3,220,832, 3,265,504, 3,276,868, 3,549,365,4,090,880, 4,153,461, 4,376,914, 4,383,987, 4,399,021, 4,427,765,4,427,766, 4,448,647, 4,452,674, 4,458,005, 4,492,616, 4,578,156,4,689,272, 4,935,332 and European Patent No. 190,643.

Radiation-sensitive layers of lithographic printing plates of thisinvention can contain a variety of optional ingredients such asantioxidants, surfactants, anti-scumming agents, and the like.

The radiation-sensitive layer can contain pigments preferably having amaximum average particle size less than about 3 micrometers. Thesepigments can provide a visible coloration to an image before or afterdevelopment of the plate. Useful pigments are well known in the art andinclude titanium dioxide, zinc oxide, copper phthalocyanines,halogenated copper phthalocyanines, quinacridine, and colorants such asthose sold commercially under the trade name Hostaperm. The pigments aregenerally present in the composition in an amount within the range offrom 0 to about 50 percent (by weight) based on the total drycomposition weight. Preferred amounts are within the range of from about5 to about 20 percent (by weight).

The exposed printing plate can be developed by flushing, soaking,swabbing or otherwise treating the radiation-sensitive layer with asolution (hereinafter referred to as a developer) which selectivelysolubilizes (i.e., removes) the unexposed areas of theradiation-sensitive layer. The developer is preferably an aqueoussolution having a pH as near to neutral as is feasible.

In a preferred form, the developer includes a combination of water andan alcohol that is miscible with water, or able to be rendered miscibleby the use of cosolvents or surfactants, as a solvent system theproportions of water and alcohol can be varied widely but are typicallywithin the range of from 40 to 99 percent by volume water and from 1 to60 percent by volume alcohol. Most preferably, the water content ismaintained within the range of from 60 to 90 percent by volume. Anyalcohol or combination of alcohols that does not chemically adverselyattack the radiation-sensitive layer during development and that ismiscible with water in the proportions chosen for use can be employed.Exemplary of useful alcohols are glycerol, benzyl alcohol,2-phenoxy-ethanol, 1,2-propanediol, sec-butyl alcohol and ethers derivedfrom alkylene glycols--i.e., dihydroxy poly(alkylene oxides)--e.g.,dihydroxy poly(ethylene oxide), dihydroxy poly(propylene oxide), etc.

It is recognized that the developer can, optionally, contain additionaladdenda. For example, the developer can contain dyes and/or pigments. Itcan be advantageous to incorporate into the developer anti-scummingand/or anti-blinding agents as is well recognized in the art.

After development, the printing plate can be treated in any known mannerconsistent with its intended use. For example, lithographic printingplates are typically subjected to desensitizing etches.

In the photopolymerizable compositions of this invention, the high levelof sensitivity, on the order of 0.1 mJ/cm², is achieved by means of thechemical amplification effects provided by free-radical initiatedpolymerization of addition-polymerizable ethylenically-unsaturatedcompounds. The propagation of radical reactions is quenched by oxygen,so it is necessary to provide the photopolymerizable coating with anoxygen barrier.

Thus, the lithographic printing plates of this invention preferablyinclude an oxygen barrier layer overlying the radiation-sensitive layer.This oxygen barrier layer serves to prevent quenching of initiating andpropagating radicals involved in the photopolymerization process.Particularly effective oxygen barrier layers are those composed ofpolyvinyl alcohol.

Polyvinyl alcohols suitable for use as oxygen barrier layers are wellknown commercially available materials. They preferably have an averagemolecular weight in the range of from about 3,000 to about 120,000.Examples of suitable polyvinyl alcohols include those available in arange of molecular weights from AIR PRODUCTS CORPORATION under thetrademarks AIRVOL 107, AIRVOL 203, AIRVOL 205, AIRVOL 523 and AIRVOL540. Other suitble polyvinyl alcohols include those available fromHOECHST-CELANESE under the trademarks MOWIOL 4-88, MOWIOL 4-98, MOWIOL5-88, MOWIOL 18-88, MOWIOL 26-88, and MOWIOL 40-88. Preferred polyvinylalcohols for use in this invention are those that are essentially fullyhydrolyzed, for example about 98% hydrolyzed, such as AIRVOL 107 orMOWIOL 4-98. Use of fully hydrolyzed polyvinyl alcohols as an oxygenbarrier is preferable to use of partially hydrolyzed polyvinyl alcoholsas they provide better protection, and consequently better photospeed,and provide better interlayer adhesion.

To form an effective oxygen barrier layer, the polyvinyl alcohol ispreferably coated in an amount of from about 0.5 to about 5 g/m² andmore preferably about 1 to about 3 g/m².

The lithographic printing plates of this invention are of highsensitivity, such as a sensitivity in the range of from about 50 toabout 500 microjoules per square centimeter. They are advantageouslydesigned to have a peak sensitivity at about 488 nanometers to adaptthem for exposure with an argon-ion laser or to have a peak sensitivityat about 532 nanometers to adapt them for exposure with a frequencydoubled Nd:YAG laser. As hereinabove described, they can be designed tohave very high sensitivities at both 488 and 532 nanometers to increasetheir versatility.

The invention is further illustrated by the following examples of itspractice taken in conjunction with the comparative examples whichillustrate the improved performance achieved with the polycarboxylicacid co-initiators of this invention. In Comparative Example A, theco-initiator is NPG whereas in Comparative Example B, it is an N-arylα-amino carboxylic acid obtained by derivatizing NPG with glycidylmethacrylate as described in Arimatsu, U.S. Pat. No. 5,378,579, to givea compound of the formula: ##STR39## This co-initiator is referred tohereinafter as Co-initiator NPG/GM.

EXAMPLE 1 AND COMPARATIVE EXAMPLES A AND B

A stock solution was prepared by admixing 3.60 parts by weight of adispersion containing binder ATAP and 20% by weight of the pigmentcopper phthalocyanine, 21.57 parts by weight of a 10% by weight solutionin 1-methoxy-2-propanol of binder ATAP, 2.16 parts by weight ofpentaerythritol triacrylate, 0.05 parts by weight of3,3'-carbonylbis(7-diethylaminocoumarin), 0.22 parts by weight ofdiphenyliodonium hexafluorophosphate, 14.16 parts by weight of2-butanone, 23.59 parts by weight of toluene and 34.33 parts by weightof 1-methoxy-2-propanol.

A coating composition, designated Composition 1, was prepared by adding0.31 parts by weight of anilinediacetic acid to 100 parts by weight ofthe above-described stock solution. Composition 1 was coated on agrained and anodized aluminum support at a dry coating weight of 1.2g/m² to form a lithographic printing plate. The printing plate was thencoated with an aqueous solution of 88% hydrolyzed poly(vinyl alcohol) ata dry coating weight of 2.2 g/m². The printing plate was exposed througha 20-step tablet on an OLEC vacuum frame, heated for 1 minute at 100°C., rinsed with water and developed with the developing compositiondescribed in Example 1 of copending commonly assigned U.S. patentapplication Ser. No. 268,100, filed Jun. 29, 1994, "Aqueous DeveloperFor Lithographic Printing Plates Which Exhibits Reduced SludgeFormation" by Gary R. Miller, John E. Walls and Melanie A. Felker nowU.S. Pat. No. 5,466,55, issued Nov. 14, 1995.

A coating composition, designated Composition A was prepared by adding0.23 parts by weight of NPG to 100 parts by weight of the stocksolution. A coating composition, designated Composition B, was preparedby adding 0.44 parts by weight of NPG/GM to 100 parts by weight of thestock solution. The amounts of NPG and NPG/GM employed were selected toprovide equimolar comparisons with the anilinediacetic acid used inComposition 1. Compositions A and B were used to prepare lithographicprinting plates in the identical manner to Composition 1 and the plateswere exposed and processed in the identical manner.

Optical densities of the image of the step tablet were converted intocontrol rating units wherein a difference of 30 units corresponds to afactor of two difference in photospeed. The plates were subjected toaccelerated aging at a temperature of 50° C. and the photospeeds weredetermined at designated intervals. The results obtained are summarizedin Table 1 below.

                  TABLE 1                                                         ______________________________________                                                 Control Rating Units                                                                        Comparative                                                                             Comparative                                  Days at 50° C.                                                                    Example 1   Example A Example B                                    ______________________________________                                        0          261         300       345                                          1          244         253       218                                          2          247         236       240                                          4          239         227       220                                          7          238         198       199                                          14         237         163       175                                          ______________________________________                                    

As shown by the data in Table 1, 14 days of incubation at 50° C. reducedthe photospeed of Example 1 by only 1.7 times whereas it reduced thephotospeed of Comparative Example A by 23 times and it reduced thephotospeed of Comparative Example B by 50 times. Thus, the printingplate of Example 1 exhibits greatly superior shelf-life. While theplates of Comparative Examples A and B have high initial photospeeds,they rapidly lose speed and thus in terms of useful photospeed aregreatly inferior.

EXAMPLE 2

A lithographic printing plate similar to that of Example 1 was preparedin which the grained and anodized aluminum support was coated with aradiation-sensitive layer comprising 140 mg/m² of the copperphthalocyanine pigment dispersion, 410 mg/m² of binder ATAP, 210 mg/m²of pentaerythritol triacrylate, 150 mg/m² of an alkoxylated triacrylate(available from SARTOMER CORPORATION under the trademark SARTOMER 9008),30 mg/m² of the bis-(2-hydroxyethylmethacrylate) ester of phosphoricacid (available from CHUGAI BOYEKI CORPORATION under the trademarkKAYAMER PM-2 difunctional monomer), 10 mg/m² of Sensitizer A, 10 mg/m²of Sensitizer B, 70 mg/m² of anilinediacetic acid, 50 mg/m² ofdiphenyliodonium hexafluorophosphate and 40 mg/m² ofbis-(η5-2,4-cyclopentadienyl-1-yl)bis2,6-difluoro-3-(1H-pyrrol-1-yl)phenyl!titanium (available fromCIBA-GEIGY CORPORATION under the trademark CGI-784). The printing platewas then coated with an aqueous solution of 98% hydrolyzed poly(vinylalcohol) at a dry coating weight of 2.2 g/m².

The lithographic printing plate was exposed, processed and evaluated inthe manner described in Example 1. The value for initial photospeed,converted into control rating units, was 306, indicating that the plateexhibited high photospeed.

EXAMPLE 3

A lithographic printing plate similar to that of Example 1 was preparedin which the grained and anodized aluminum support was coated with aradiation-sensitive layer comprising 140 mg/m² of the copperphthalocyanine pigment dispersion, 480 mg/m² of binder ATAP, 480 mg/m²of pentaerythritol triacrylate, 10 mg/m² of Sensitizer A and 69 mg/m² ofanilinediacetic acid. The lithographic printing plate was exposed,processed and evaluated in the manner described in Example 1 and theinitial photospeed, converted into control rating units, was determinedto be 211.

EXAMPLE 4

A lithographic printing plate was prepared as in Example 3 except thatit additionally contained 48 mg/m² of diphenyliodoniumhexafluorophosphate. The control rating unit values were 272 for initialphotospeed and 246 for photospeed after 14 days at 50° C.; indicatingthat the addition of an iodonium salt to the composition of Example 3substantially improved photospeed.

COMPARATIVE EXAMPLE C

A lithographic printing plate was prepared as in Example 3 except thatit additionally contained 45 mg/m² of the diazonium salt4-diazo-3-methoxydiphenylamine hexafluorophosphate. The control ratingunit value for initial photospeed was determined to be 115 indicatingthat the addition of this diazonium salt brought about a major reductionin photospeed of the composition of Example 3.

COMPARATIVE EXAMPLE D

A lithographic printing plate was prepared as in Example 3 except thatit additionally contained 59 mg/m² of a mixture of triphenylsulfoniumhexafluorophosphate and diphenyl phenylthiophenyl!-sulfoniumhexafluorophosphate (available from Minnesota Mining and ManufacturingCompany under the trademark FX-512). The control rating unit value forinitial photospeed was determined to be 210 indicating that the additionof this sulfonium salt did not improve the photospeed of the compositionof Example 3.

EXAMPLE 5

A lithographic printing plate was prepared as in Example 3 except thatit additionally contained 53 mg/m² of2,4,6-tris(trichloromethyl)-s-triazine. The control rating unit valueswere 249 for initial photospeed and 233 for photospeed after 14 days at50° C., indicating that the addition of a triazine to the composition ofExample 3 substantially improved photospeed.

EXAMPLE 6

A lithographic printing plate was prepared as in Example 3 except thatit additionally contained 33 mg/m² of N-methoxy-4-phenylpyridiniumtetrafluoroborate. The control rating unit values were 246 for initialphotospeed and 209 for photospeed after 14 days at 50° C. indicatingthat the addition of an N-alkoxypyridinium salt to the composition ofExample 3 substantially improved photospeed.

EXAMPLE 7

A lithographic printing plate was prepared as in Example 3 except thatit additionally contained 81 mg/m² of2,2'-bis(o-chlorophenyl)-4,5,4',5'-tetraphenyl-1,2'-biimidazole. Thecontrol rating unit values were 242 for initial photospeed and 192 forphotospeed after 14 days at 50° C., indicating that the addition of ahexaarylbisimidazole to the composition of Example 3 substantiallyimproved photospeed.

EXAMPLES 8 AND 9 AND COMPARATIVE EXAMPLES E AND F

Coating compositions as described in Table 2 below were prepared andcoated on grained and anodized aluminum supports to form printingplates. The printing plates were exposed, processed and evaluated in thesame manner as described in Example 1.

                  TABLE 2                                                         ______________________________________                                                 Parts By Weight                                                                                   Comparative                                                                           Comparative                              Component  Example 8                                                                              Example 9                                                                              Example E                                                                             Example F                                ______________________________________                                        Copper      2.97     2.97     2.97    2.97                                    phthalocyanine                                                                dispersion.sup.(1)                                                            Acetal polymer.sup.(2)                                                                   20.82    20.82    20.82   20.82                                    Pentaerythritoltri-                                                                       2.19     2.19     2.19    2.19                                    acrylate                                                                      Ethyl Eosin                                                                               0.11     0.11     0.11   0                                        Diphenyliodonium                                                                          0.21    0         0.21                                            hexafluorophosphate                                                           Anilinediacetic acid                                                                      0.26     0.26    0        0.26                                    2-Butanone 13.83    13.86    13.87   13.84                                    Toluene    23.05    23.10    23.11   23.07                                    1-Methoxy- 36.45    36.58    36.61   36.52                                    2-propanol                                                                    ______________________________________                                         .sup.(1) A dispersion containing binder ATAP and 20% by weight of copper      phthalocyanine.                                                               .sup.(2) A 10% by weight solution in 1methoxy-2-propanol of binder ATAP. 

For each of the printing plates, the optical densities of the image ofthe step tablet were converted into control rating units and the resultsobtained are summarized in Table 3 below.

                  TABLE 3                                                         ______________________________________                                               Control Rating Units                                                                                Comparative                                                                           Comparative                              Days at 50° C.                                                                  Example 8 Example 9 Example E                                                                             Example F                                ______________________________________                                         0       162       93        97      0                                         1       162       84        98      0                                         5       161       86        74      0                                         7       161       89        60      0                                        14       150       78        71      0                                        ______________________________________                                    

The results reported in Table 3 indicate that ethyl Eosin, a xanthenedye, is effective as a sensitizer in the present invention. In Example 8anilinediacetic acid was used as a first co-initiator anddiphenyliodonium hexafluorophosphate was used as a second co-initiatorand both high photospeed and good shelf-life were obtained. Example 9differed from Example 8 in that the diphenyliodonium hexafluorophosphatewas omitted and as a result the photospeed was significantly lower.Comparative Example E differed from Example 8 in that theanilinediacetic acid was omitted and as a result both photospeed andshelf-life were significantly lower. Comparative Example F differed fromExample 8 in that the sensitizer was omitted with the result that noimage was obtained under the exposure conditions employed.

The data in Table 3 indicate that both anilinediacetic acid anddiphenyliodonium hexafluorophosphate are reasonably effective in meetingthe dual objectives of high photospeed and good shelf-life but that whenthey are used in combination as in Example 8 much better results areachieved.

EXAMPLE 10

A lithographic printing plate similar to that of Example 1 was preparedin which the grained and anodized aluminum support was coated with aradiation-sensitive layer comprising 140 mg/m² of the copperphthalocyanine pigment dispersion, 410 mg/m² binder ATAP, 325 mg/m² ofpentaerythritol triacrylate, 150 mg/m² of an alkoxylated triacrylate(available from SARTOMER CORPORATION under the trademark SARTOMER 9008),25 mg/m² of the bis (2-hydroxyethyl methacrylate ester of phosphoricacid (available from CHUGAI BOYEKI CORPORATION under the trademarkKAYAMER PM-2 difunctional monomer), 10 mg/m² of Sensitizer A, 33 mg/m²of N-methoxy-4-phenylpyridinum tetrafluoroborate, 70 mg/m² ofanilinediacetic acid and 40 mg/m² of bis(↓⁵ -2,4-cyclopentadien-1-yl)bis2,6-difluoro-3-(1H-pyrrol-1-yl)phenyl!-titanium. The printing plate wasthen coated with an aqueous solution of 98% hydrolyzed poly(vinylalcohol) at a dry coating weight of 2.2 g/m².

The lithographic printing plate was exposed, processed and evaluated inthe manner described in Example 1. For each sample of the printingplate, the optical densities of the image of the step tablet wereconverted into control rating units and the results obtained aresummarized in Table 4 below.

                  TABLE 4                                                         ______________________________________                                                      Days at 38° C.                                                         80% Relative                                                                             Control Rating                                       Days at 50° C.                                                                       Humidity   Units                                                ______________________________________                                         0            --         273                                                   3            --         257                                                   7            --         259                                                  14            --         260                                                  --            7          257                                                  ______________________________________                                    

The results reported in Table 4 indicate that the formulation of Example10 provided both high photospeed and good shelf-life.

EXAMPLE 11

A lithographic printing plate similar to that of Example 10 was preparedin which the 33 mg/m² pf N-methoxy-4-phenylpyridinium tetrafluoroboratewas replaced with 36 mg/m² of 1-methoxy-3-(methoxycarbonyl)-pyridiniumhexafluorophosphate. The control rating unit value was 271 for theinitial photospeed.

EXAMPLE 12

A lithographic printing plate similar to that of Example 10 was preparedin which the 33 mg/m² of N-methoxy-4-phenylpyridinium tetrafluoroboratewas replaced with 46 mg/m² of1-methoxy-3-(2-phenyl-1-ethoxycarbonyl)pyridinium hexafluorophosphate.The control rating unit value was 278 for the initial photospeed.

EXAMPLE 13

A lithographic printing plate similar to that of Example 10 was preparedin which the 33 mg/m² of N-methoxy-4-phenylpyridinum tetrafluoroboratewas replaced with 36 mg/m² of 1-methoxy-4-cyanopyridiniumtetrafluoroborate. The control rating unit value was 298 for the initialphotospeed.

EXAMPLE 14

A lithographic printing plate similar to that of Example 10 was preparedin which the 33 mg/m² of N-methoxy-4-phenylpyridinium tetrafluoroboratewas replaced with 30 mg/m² of 1-methoxypyridinium hexafluorophosphate.The control rating unit value was 299 for the initial photospeed.

EXAMPLE 15

A lithographic printing plate similar to that of Example 2 was preparedin which the 70 mg/m² of anilinediacetic acid was replaced with 70 mg/m²of 2- (carboxymethyl)methylamino!benzoic acid.

The lithographic printing plate was exposed, processed and evaluated inthe manner described in Example 1. The values for initial photospeed andphotospeed after 14 days at 50° C., converted to control rating units,were 281 and 283, respectively, indicating that the plate exhibited bothhigh photospeed and good shelf-life.

EXAMPLE 16

A lithographic printing plate similar to that of Composition 1 inExample 1 was prepared in which the 0.31 parts by weight ofanilinediacetic acid was replaced with an equimolar amount ofp-chlorophenyl-iminodiacetic acid.

The lithographic printing plate was exposed, processed and evaluated inthe manner described in Example 1. The value for initial photospeed,converted to control rating units, was 268.

EXAMPLE 17

A lithographic printing plate similar to that of Example 2 was preparedin which the 50 mg/m² of diphenyliodonium hexafluorophosphate wasreplaced with 70 mg/m² of 4-(octyloxyphenyl)phenyliodonium tosylate.

The lithographic printing plate was exposed, processed and evaluated inthe manner described in Example 1. The values for initial photospeed andphotospeed after 14 days at 50° C., converted to control rating units,were 309 and 287, respectively, indicating that the plate exhibited bothhigh photospeed and good shelf-life.

EXAMPLE 18

A lithographic printing plate similar to that of Composition 1 inExample 1 was prepared in which the pentaerythritol triacrylate wasreplaced with an equal amount of the reaction product ofhexamethylene-diisocyanate with two equivalents of 2-hydroxyethylmethacrylate.

The lithographic printing plate was exposed, processed and evaluated inthe manner described in Example 1. The value for initial photospeed,converted to control rating units, was 301.

EXAMPLE 19

A lithographic printing plate was prepared as in Example 3 except thatit additionally contained 70 mg/m² of tetramethylammoniumn-butyltriphenylborate and 50 mg/m² of2-(1-naphthalenyl)-4,6-bis(trichloromethyl)-1,3,5-triazine. The controlrating unit value was 249 for initial photospeed and 173 after sevendays of incubation at 50° C.

EXAMPLE 20

Lithographic printing plates similar to that of Example 1 were preparedin which the grained and anodized aluminum supports were coated to a dryweight of about 1.1 g/m² with the radiation-sensitive formulationsdescribed in Table 5 below. Each plate was also coated with an aqueoussolution of 98% hydrolyzed poly(vinyl alcohol) at a dry coating weightof 2.2 g/m².

                  TABLE 5                                                         ______________________________________                                        Ingredient         A      B       C    D                                      ______________________________________                                        20% copper phthalocyanine pigment                                                                 3.33   3.33    3.33                                                                               3.33                                  dispersion                                                                    10% ternary vinylacetal binder resin                                                             18.07  18.07   18.07                                                                              18.07                                  pentaerythritol triacrylate                                                                       1.33   1.33    1.33                                                                               1.33                                  SARTOMER 9008       0.67   0.67    0.67                                                                               0.67                                  KAYAMER PM-2        1.11   1.11    1.11                                                                               1.11                                  3,3'-Carbonylbis(5,7-dipropoxy-                                                                   0.04   0.04    0.04                                                                               0.04                                  coumarin)                                                                     co-initiator        0.00   0.33    0.35                                                                               0.31                                  2-butanone         14.29  14.24   14.24                                                                              14.25                                  toluene            23.82  23.74   23.74                                                                              23.75                                  1-methoxy-2-propanol                                                                             37.33  37.13   37.12                                                                              37.15                                  ______________________________________                                    

Control formulation A had no co-initiator, resorcinol-O,O'-diacetic acidwas the co-initiator for formulation B, 4-methylcatechol-O,O'-diaceticacid was the co-initiator for formulation C, and2-(carboxymethylthio)benzoic acid was the co-initiator for formulationD.

Each lithographic printing plate was exposed, processed and evaluated inthe manner described in Example 1. The value for the initialphotospeeds,. converted into control rating units, is tabulated below.

    ______________________________________                                                   A    B          C      D                                           ______________________________________                                        Photospeed   136    151        143  146                                       ______________________________________                                    

The above results show that the coatings with the added co-initiatorhave greater photospeeds than the comparison A lacking any co-initiator.

The invention has been described in detail, with particular reference tocertain preferred embodiments thereof, but it should be understood thatvariations and modifications can be effected within the spirit and scopeof the invention.

What is claimed is:
 1. A photopolymerizable composition comprising:(1)at least one addition-polymerizable ethylenically-unsaturated compound,and (2) a photopolymerization initiator system comprising:(A) a spectralsensitizer that sensitizes in the ultraviolet or visible regions of thespectrum, and (B) a first co-initiator;wherein said spectral sensitizeris: ##STR40## or a mixture thereof; and wherein the first co-initiatoris: ##STR41## wherein n is an integer of from 1 to 5, m is an integer offrom 1 to 5, and R is hydrogen or an alkyl group of 1 to 6 carbon atoms;or ##STR42## wherein Q¹ and Q² are independently hydrogen or halogen;and wherein said photopolymerizable composition is photoimagable.
 2. Thephotopolymerizable composition of claim 1 in which the initiator systemadditionally comprises a second co-initiator selected from the groupconsisting of iodonium salts, titanocenes, haloalkyl-substituteds-triazines, hexaaryl bisimidazoles, alkyltriarylborate salts andphotooxidants containing a heterocyclic nitrogen atom that issubstituted by either an alkoxy group or an acyloxy group.
 3. Thephotopolymerizable composition of claim 2 in which the a secondco-initiator is a diaryl iodonium salt.
 4. The photopolymerizablecomposition of claim 1 in which the first co-initiator is selected fromthe group consisting of anilinediacetic acid,p-chlorophenyl-iminodiacetic acid, and2-((carboxymethyl)methylamino)benzoic acid.
 5. The photopolymerizablecomposition of claim 4 in which the initiator system additionallycomprises a second co-initiator selected from the group consisting ofiodonium salts, titanocenes, haloalkyl-substituted s-triazines, hexaarylbisimidazoles, alkyltriarylborate salts and photooxidants containing aheterocyclic nitrogen atom that is substituted by either an alkoxy groupor an acyloxy group.
 6. The photopolymerizable composition of claim 5 inwhich the first co-initiator is anilinediacetic acid.
 7. Thephotopolymerizable composition of claim 6 in which the a secondco-initiator is a diaryl iodonium salt.
 8. The photopolymerizablecomposition of claim 7 in which the addition-polymerizableethylenically-unsaturated compound is a polyfunctional acrylic monomer.9. A photopolymerizable composition comprising:(1) at least oneaddition-polymerizable ethylenically-unsaturated compound, and (2) aphotopolymerization initiator system comprising:(A) a spectralsensitizer that sensitizes in the ultraviolet or visible regions of thespectrum, the spectral sensitizer being a coumarin, fluorescein,fluorone, xanthene, merocyanine, thioxanthone or isoalloxanine compound,and (B) a first co-initiator, wherein the first co-initiator is:##STR43## wherein n is an integer of from 1 to 5, m is an integer offrom 1 to 5, and R is hydrogen or an alkyl group of 1 to 6 carbon atoms;or ##STR44## wherein Q¹ and Q² are independently hydrogen or halogen;and wherein said photopolymerizable composition is photoimagable. 10.The photopolymerizable composition of claim 9 in which the firstco-initiator is selected from the group consisting of anilinediaceticacid, p-chlorophenyl-iminodiacetic acid, and2-((carboxymethyl)methylamino)benzoic acid.
 11. The photopolymerizablecomposition of claim 9 in which the first co-initiator isanilinediacetic acid.
 12. The photopolymerizable composition of claim 9in which the initiator system additionally comprises a secondco-initiator selected from the group consisting of iodonium salts,titanocenes, haloalkyl-substituted s-triazines, hexaaryl bisimidazoles,alkyltriarylborate salts and photooxidants containing a heterocyclicnitrogen atom that is substituted by either an alkoxy group or anacyloxy group.
 13. The photopolymerizable composition of claim 12 inwhich the first co-initiator is anilinediacetic acid.
 14. Thephotopolymerizable composition of claim 13 in which the secondco-initiator is a diaryl iodonium salt.
 15. The photopolymerizablecomposition of claim 14 in which the spectral sensitizer is a3-substituted coumarin.
 16. The photopolymerizable composition of claim13 in which the addition-polymerizable ethylenically-unsaturatedcompound is a polyfunctional acrylic monomer.
 17. The photopolymerizablecomposition of claim 16 additionally comprising a film-forming polymericbinder.
 18. The photopolymerizable composition of claim 17 in which thesecond co-initiator is a diaryl iodonium salt and in which the spectralsensitizer is a 3-substituted coumarin.
 19. The photopolymerizablecomposition of claim 17 in which the second co-initiator is anN-alkyoxypyridinum salt.
 20. The photopolymerizable composition of claim19 in which the spectral sensitizer is a 3-substituted coumarin.
 21. Thephotopolymerizable composition of claim 12 in which the compositionexhibits superior shelf life to compositions containing N-phenylglycineas the first co-initiator.
 22. The photopolymerizable composition ofclaim 21 in which the first co-initiator is anilinediacetic acid.